T>his tracking bench supply is built almost entirely from standard components but provides high performance. It is a linear supply which offers very good regulation and low ripple and noise (see specifications table). It also boasts a digital display for voltage and current readouts and this can also show the voltage across both rails or the present current-limit setting.
Fig.1: this block diagram shows the basic operation of the supply. Variable regulators REG1 & REG2 provide the positive and negative output rails, while IC4b ensures that REG2 tracks REG1. R1, R2, IC1 & IC2 monitor the rail currents and provide limiting as necessary.
The primary outputs track each other, providing balanced rails, or a load can be connected across both to double the voltage. Either way, the current limit can be adjusted from 0-1.6A (0-1.0A for the plugpack version). The internal regulators are protected against excessive temperature or current.
A third output supplies a fixed 5.0V at up to 750mA. The supply also incorporates an earth terminal, a load switch (which controls all three outputs) and a power switch.
This supply is particularly well suited for breadboarding, especially for circuits which mix digital logic and analog signal processing. If you prototype this type of circuit often you will be familiar with the hassles involved with building a power supply each time which is able to deliver 5V and/or 3.3V, along with balanced rails (eg, ±15V) for the op amps.
With a tracking supply such as this one, not only is most of that effort spared but you can easily observe the current consumed by the op amps and set the current limit to a suitable level so that a wiring mistake in the prototype will not cause any damage.
We have tried to keep the cost and complexity down as much as possible while providing several improvements over our original Dual Tracking Supply which we featured way back in the January 1988 issue. The improvements include current readout, adjustable current limit, fixed 5V output, digital display, a voltage measurement across both outputs, a larger transformer and the plugpack supply option.
Construction is simplified by mount-
ing most of the front panel components on a second PC board. This is connected to the main PC board via several ribbon cables and a few heavy duty wires.
While all of the parts can be obtained from virtually any large electronics retailer, the 0.1Ω 5W shunt resistors can be replaced with less common 1% types (or better) for improved current measurement accuracy. Alternatively, use a millivoltmeter to test a number of 5% resistors for accuracy. We chose two at random for our first prototype and as luck would have it, they were within 1%.
Because this is a tracking supply, under normal conditions, the absolute voltage at the negative output matches that of the positive output. In other words, if the positive output is adjusted to +9.3V, the negative output will be -9.3V. As a result, only one voltage adjustment knob is required. Many circuits, especially those with op amps, work best with balanced rails.
The 5.0V output is supplied by a 7805T regulator, which has its own current and thermal limiting. This rail also powers the panel meter and power LED, so if you manage to short the output, it will be obvious! It’s best to avoid shorting it if possible but if the display goes blank, disconnect the 5.0V output (or clear the short) to restore it.